The present invention relates to a high-transverse-curvature tire, in particular for motor-vehicle wheels, comprising: a carcass structure consisting of at least one carcass ply having its circumferential end edges turned back around two bead cores; a belt structure extending circumferentially about the carcass ply and having at least one circumferentially inextensible cord wound in a plurality of circumferential coils disposed side by side to define at least one layer of a curvilinear transverse profile; a tread band applied circumferentially to the belt structure; said inextensible cord being wound on an auxiliary support element made in the form of an elastomeric sheet, interposed between the belt structure and carcass ply and shaped according to the transverse-curvature profile of the layer formed by said coils.
While being conceived in particular with reference to the manufacture of tires for high-performance motorcycles, the novel principles suggested by the present invention can be adopted to advantage in the manufacture of any type of tire in which the belt structure is made separately from the carcass structure and subsequently assembled, and in any case in the manufacture of any composite article in which provision is made for one or more steps requiring the use of auxiliary elements in the form of an elastomeric sheet for supporting and/or structurally stabilizing other components during manufacture.
It is known that during the manufacture of tires for motor-vehicles, or in any event tires characterized by a high transverse curvature, the belt structure is made on a so-called "comb drum", to be subsequently picked up, after application of the tread band, by a transfer ring bringing it into engagement with a carcass structure previously made on a building drum.
In greater detail, according to the traditional tire manufacturing method, for making the belt structure one or more belt strips are wound in mutual overlapping relation around the comb drum so that, when winding has been completed, said belt strips are substantially shaped as a cylindrical sleeve. At this point, upon intervention of a plurality of radially expansible sectors provided in the comb drum the belt structure acquires a curvilinear transverse profile substantially coinciding with the transverse profile that the finished tire will have.
The thus shaped belt structure is ready to be picked up from the comb drum and be coupled with the carcass structure, after application of the tread band.
One example of the above described state of the art in given in the Italian Patent Application No. 22730A/89 in the name of the same assignee, the disclosure of which is hereby incorporated by reference.
Recently, referring primarily to high-performance tires for motorcycles, belt structures which have become increasingly more used essentially consist of a single cord or a plurality of cords disposed parallelly in side by side relation to form a ribbon-like or tape structure which is directly wound on the comb drum in a plurality of circumferential parallel side by side coils substantially oriented in the rolling direction of the tire. Such a tire is disclosed for example in the Italian Patent Application No. 20646A/90 in the name of the same assignee, the disclosure of which is hereby incorporated by reference.
In this solution it is necessary that the winding operation be executed with the expansible sectors of the comb drum already radially expanded from the drum itself. In fact, since the cord used is of the inextensible type, it would be impossible to carry out the radial expansion of the sectors, if the cord had been previously wound on the comb drum in a cylindrical configuration.
However, the individual coils could encounter problems in terms of stability during winding, due to the fact that the sectors on which said cord is to be wound have a curved profile. It is therefore necessary that before the winding operation, an auxiliary support element of raw elastomeric material be laid down on the comb drum. This auxiliary element, by virtue of its adhesiveness, is capable of ensuring the geometrical and structural stability of the coil layer being progressively formed during the cord winding.
This auxiliary support element, made in the form of a sheet obtained through usual drawing and/or calendering steps, is wrapped in the form of a cylindrical sleeve around the comb drum having its sectors retracted, to be then expanded and shaped according to the the radially external profile of the expansible sectors, following the radial expansion of these latter before the beginning of the cord winding.
The fundamental properties of this support element are therefore stretchability and adhesiveness so that the element will be bound not only to the cord that is going to be wound, but also. during the tire manufacture, to the adjacent elements.
For this purpose, the use of natural rubber possessing both a good adhesiveness and a good stretchability in a raw state is therefore preferred, the bond between said support element and the adjacent elements is further promoted by the fact that also the rubberizing blends for the carcass plies and the belt strips are mainly natural-rubber-based blends.
On the other hand, this auxiliary support element does not perform any structural function, its only aim being that of stabilizing the coils formed by the cord while the belt structure is being made, so that its presence in the finished tire is not essential. It would be therefore desirable for the elastomeric sheet forming said support element to have weight and thickness as much as possible reduced in order to minimize the effects of its presence in the finished tire, in particular for the purpose of restraining the centrifugal forces induced by said support element and reducing the resistance to rolling of the tire during the running.
Furthermore, natural rubber and also other known blends, while having sufficient stretchability as a whole, are not amenable to a thickness reduction of less than 0.5 mm, due to the insufficient mechanical strength at high localized stretching variations, like those generated during the radial expansion of the curved sectors of the comb drum.
One could think about increasing this strength by means of appropriate reinforcing fillers; however known methods of imparting good mechanical strength features to an unvulcanized elastomeric material, based on the use either of previously cross-linked or crystalline polymers, or textile or mineral fibers, did not give satisfactory results: in fact they had several disadvantages, in particular a strong adhesiveness reduction often without increasing or even decreasing stretchability.
There are also other reasons preventing the sizes of said auxiliary element from being reduced under the above mentioned value: in fact, with presently used traditional blends and with usual machines, practical problems arise that virtually do not enable sheet thicknesses lower than 0.5 mm to be achieved. The following drawbacks should be noted in particular: the material has a tendency to adhere to the machine components (the calender cylinders, for example) so that it tears on separation; the sheet thickness does not keep constant but it increases on its coming out of the machine and in addition said thickness varies in an uneven manner, transversely of the feed direction of the sheet; and it is very difficult (sometimes even impossible) to collect the sheet and convey it to the subsequent use stations without causing pleats and, as a result, strong localized variations in thickness.
In accordance with the present invention, it has been found that making the auxiliary support element of an elastomeric material filled with appropriate bonding means, preferably consisting of short fibrillose fibers of poly-para-phenylene terephthalamide (usually defined as aramid pulp) commercially known as Kevlar* Pulp or Twaron** Pulp (*registered trademark of Du Pont; ** registered trademark of Akzo) in an amount between 1 and 10 phr (parts by weight per 100 parts of rubber), a surprising increase in the structural strength of the elastomeric sheet is achieved along with high adhesiveness and constancy in the imposed size values, so that it is possible to achieve greatly lower thicknesses as compared with those of the known art. The sheet thus produced is capable of bearing without damages the stresses it undergoes during the sheet preparation process, that is its winding and unwinding around and from the collecting rolls, its separation from the support means (usually polythene or other service material), its laying onto the comb drum and the subsequent stretching, by expansion of the curved sectors.
In a further aspect the invention relates to a tire of high transverse curvature, in particular for motor-vehicle wheels, characterized in that said auxiliary support element comprises fibrillose short fibers homogeneously distributed in the elastomeric material so as to increase the stretchability features of said sheet without substantially altering the adhesiveness features of said elastomeric material in the raw state.
Preferentially, said fibrillose short fibers comprise short fibers of poly-para-phenylene terephthalamide, inserted in the elastomeric material forming said sheet in an amount between 1 and 10 phr and having a length between 0.1 mm and 2.5 mm.
Advantageously, said sheet of elastomeric material has a thickness between 0.075 and 0.5 mm.
Preferably, said fibrillose short fibers are pre-oriented (by a calendering operation for example) in the major direction of the forces to which the support element is submitted during the tire manufacture process. Normally this direction is the circumferential direction of the tire and said pre-orienting operation is preferentially carried out by calendering of said sheet during the manufacture process of same.
The elastomeric material reinforced with said aramid, in the raw state has a tensile strength included between 3 and 7 MPa and a 50% elongation under a traction load between 0.2 and 0.5 MPa.
Preferentially the material forming said auxiliary support element is a natural rubber-based blend containing carbon black in an amount between 30 and 70 phr, filled with usual ingredients known in the art (plasticizers, protection agents, degradation-resistent and vulcanizing agents) so as to obtain an elastomeric matrix as much as possible similar to that of the elements to which said sheet must adhere.
The invention also relates to a process for manufacturing said tire, and the auxiliary support element as such, to be generally used for structural stabilization of constructional components in a composite manufactured article, in the intermediate manufacturing steps of same.